At present time, as a negative electrode material for a lithium battery that is generally used, a carbon material is cited. Since the carbon material has an intercalation reaction, though excellent in the cycle retention rate, it is considered difficult to obtain high capacity. Accordingly, a negative electrode material that can replace the carbon material and has high capacity is under active study.
As a high capacity negative electrode material, low-cost α-Fe2O3 that places little burden on the environment is proposed. The α-Fe2O3 causes a reaction called a conversion electrode reaction expressed by the following formula (1); accordingly, a theoretical capacity thereof is such high as 1007 mAh/g that is about 3 times larger than that of the carbon material that is a conventional negative electrode material.
[Chemical 1]Fe2O3+6Li++6e− 2Fe+3Li2O  (1)
However, when the α-Fe2O3 is used as a negative electrode material, in association with the electrode reaction, a volume expansion of about 2 times occurs. As the result thereof, there are various problems that by the volume expansion of α-Fe2O3, an electrode layer is peeled off a boundary between a negative electrode current collector and a negative electrode layer or an electrode structure is broken to result in an irregular electrode reaction, further, because of the use α-Fe2O3a as the negative electrode material, a first time irreversible capacity becomes large and the cycle deterioration is large.
For example, when a negative electrode is formed generally by use of a coating method, firstly, a negative electrode active material, a conductive assistant, and a binder solution are mixed to prepare a paste having proper viscosity. Usually, as the conductive assistant, conductive carbon is used, and as the binder, PVdF (polyvinylidene fluoride) is used. The prepared paste is thinly coated on a copper foil that is used as a current collector, and, after heating in vacuum at 120 to 170° C., dried and pressed. Thereby, a negative electrode composed of the current collector and an electrode layer formed on the current collector is prepared.
Here, when α-Fe2O3 is used as a negative electrode active material, according to conventional heat treatment, the binding force of PVdF used as the binder is not enough. Accordingly, because of large volume expansion of α-Fe2O3 generated during charge and discharge, an electrode layer is peeled off a boundary between the current collector and the electrode layer, or the electrode layer is excessively cracked to result in remarkable capacity deterioration within several cycles.
Further, it is also known that the PVdF used as the binder swells owing to permeation of an electrolytic solution. Specifically, it is generally known that the electrode layer swells by about 10% in a thickness direction to a thickness of the electrode layer before immersing the electrode in the electrolytic solution. It is feared that according to the phenomenon like this, the binding property may be insufficient to further promote the breakdown of the electrode. Still further, there was a problem that owing to the swelling of the binder, contact (adhesion) between the current collector and active material powder or contact between the active material powders with each other are deteriorated to result in being incapable of exhibiting enough performance.
In order to solve the above described problems, there are trials that α-Fe2O3a is nano-particulated or micro-particulated to improve the cycle characteristics (see, for example, Patent Document 1) or an electrode sheet is heated at 300° C. or more to enhance the binding property of the binder.
However, according to the former method described in the Patent Document 1, there were problems that owing to aggregation of particles, an electrode is difficult to prepare, owing to a decrease in an electrode reaction area, the capacity decreases, and owing to an excessive electrochemical reaction with an electrolytic solution, the capacity decreases.
Further, according to the latter method, a material such as copper, which is used as a current collector tends to be oxidized. Accordingly, it is necessary to conduct a heat treatment under, for example, an oxygen-free argon gas atmosphere. As the result thereof, there is a problem of an increase in the production cost. Further, even when the binding property of the binder is enhanced according to the method, the cycle characteristics could not be said sufficiently improved.
As obvious from what was mentioned above, it was difficult to sufficiently derive performance of α-Fe2O3 according to a conventional electrode layer constituent material, in particular, a binder, or according to a conventional method for producing an electrode.
In addition, a method of producing a binder that is used in place of the conventional PVdF, in which, when a binder polymer composed of a polyester amide oligomer containing two or more functional groups selected from a carboxyl group, an amino group and a hydroxyl group in a molecular chain and diisocyanate is dissolved and dispersed in N-methyl-2-pyrolidone, the binder polymer is, after heating at 140 to 180° C. for 1 to 3 hr to dissolve, under stirring, cooled at a rate of temperature decrease of 2 to 10° C./hr to produce a binder is disclosed (see, for example, Patent Document 2).
It is said that the binder obtained according to the method described in the Patent Document 2 does not precipitate when mixed with an active material, does not crack an electrode coating film, has excellent adhesive force with a metal foil, does not change the viscosity of the solution over a long period of time, does not adversely affect on the coating property during production of batteries, can suitably use as a mobile power supply for such as portable electronic equipments and so on, has high capacity and is excellent in the charge/discharge characteristics, and is high in the capacity retention rate even after repetitive use over a long term.